1. Field of the Invention
The present invention generally relates to heat-assisted magnetic recorders and, more particularly, to a magnetic recorder having carbon nanotubes (CNTs) embedded in an anodic alumina template for emitting electron beams onto a magnetic recording medium in order to perform heat-assisted magnetic recording of the medium.
2. Background Art
Magnetic recording mediums for recording information have a magnetic layer comprised of minute magnetic grains. Magnetic domains recorded on the magnetic layer have to be small enough to accommodate high-density recording carried out by a magnetic recording head. In order to enable clear distinction of small recording magnetic domains, the boundaries of the domains should be smooth enough, and this results in the reduction of the grain size. The propagation of magnetization reversal among the grains distorts the domain boundary. Thus, individual grains can be isolated magnetically by nonmagnetic substances to prevent exchange coupling interaction among the grains. Moreover, from the viewpoint of the magnetic interaction between the head and the medium, the magnetic layer has to be thin enough for the high-density recording.
To satisfy those requirements, the volume of the magnetization reversal unit (whose size becomes almost equal to that of a grain as more of those requirements are met) has to decrease as the density increases. However, if the volume of the magnetization reversal unit is diminished, then the magnetic anisotropy energy of the unit (Ku (density of magnetic anisotropy energy) * V (volume of the magnetization reversal unit)) becomes smaller than the thermal fluctuation energy, and it will be no longer possible to maintain the domains. This is the thermal fluctuation phenomenon. The physical limit of recording density governed by this phenomenon is called the thermal fluctuation limit or the superparamagnetic limit.
For shrinking magnetic grain volumes, magnetization reversal by thermal fluctuation can be prevented if the magnetic anisotropy energy density (Ku) is increased. However, because the recording coercive field (Hc) of the magnetization in the recording medium is substantially proportional to Ku, a magnetic recording field at or above Hc is required for achieving a sufficient recording in the medium. In this connection, it is noted that magnetic recording head characteristics which determine the magnetic recording field are rapidly reaching their physical limit, making it unreasonable to expect a further improvement in the magnetic recording field. That is, it is becoming increasingly difficult to comply with the demands for the increased recording density by simply increasing Ku.
Accordingly, recording onto a medium at room temperature and a large Ku value is not possible because the magnetic field intensity necessary for recording exceeds the intensity of a magnetic recording field generated by a magnetic head. This problem of recording to mediums having relatively large Ku values is solved by the use of so-called “heat-assisted magnetic recording” systems. In a heat-assisted magnetic recording system, a heater locally heats a medium during recording in order to lower the recording coercive field (Hc) of the medium lower than the intensity of the magnetic recording field generated by a head. As a result, the head is able to record information on the medium at the heated location.
A laser beam generator is typically used as a heater in heat-assisted magnetic recording systems. A laser beam generator directs a laser beam onto the medium in order to locally heat the medium where data bits are to be recorded by the head. The problem with using a laser beam generator to locally heat a medium is that it is difficult to efficiently deliver laser beam power to a relatively small area of the medium. The area of the medium to be heated is relatively small because the sizes of the data bits to be recorded to this area are correspondingly small (on the order of 20 nm to 50 nm). As such, the data bit sizes are significantly below the diffraction limit of a typical laser beam generator. For example, the diffraction limit for a 500 nm light with a numerical aperture of one is 305 nm. As a result, a diffraction limited laser beam undesirably heats adjacent tracks of the medium.
Smaller areas can be heated with a laser through the use of near-field optics. In near-field optics, only a small percentage of the total light is incident in the desired area to be heated. Efficiencies for near-field optics are correspondingly small and the energy not transmitted to the medium must be removed from the head somehow. The subsequent heating of the head can be problematic for actual devices.